Top 4 Lightweight, Official Ubuntu Based Alternatives for Ubuntu 11.04's Unity

"Unity 2D's goal is to provide the Unity desktop shell on
hardware platforms that cannot currently support Unity's OpenGL
requirements. Simply put, Unity 2D is a lightweight Unity interface
that can run smoothly on non accelerated hardwares. Unity 2D is not
a default in Ubuntu 11.04 Natty Narwhal and is very much a work in
progress. But from Ubuntu 11.10 Oneiric Ocelot onwards, Unity 2D
will replace classic GNOME as the default fall back desktop."